The following text has been excerpted and modified with permission from copyrighted material. Material in this section should not be reproduced in any manner without permission of the North American Lake Management Society (www.NALMS.org). The reader is encouraged to read the original material for more details on trophic state theory and methods. Material should be cited as:
Carlson, R.E. and J. Simpson. 1996. A Coordinator’s Guide to Volunteer Lake Monitoring Methods. North American Lake Management Society. 96 pp.
314 of the Clean Water Act requires that all lakes of the Nation be
classified according to their “eutrophic” character. Federal
requirements for the Section 305b of the same Act requires that
“fishable” and “swimmable” goals be constructed for each
state. Federal requirements such as these have resulted in the
construction and use of numerous classification schemes (Brezonik and
Shannon 1971; Carlson 1977; USEPA. 1974; Vollenweider and Kerekes
1980). These indices vary considerably in approach to classification
and in the variables used for classification. All of these indices are
“trophic state indices.” They are predicated on the ideas that:
(1) the “trophic state” of a lake is an important water quality
variable; and (2) indices can determine not only the trophic character
of the water body, but also if it is usable for fishing or swimming.
If trophic state classification is as valuable as the interest in it suggests, then it would be advantageous if volunteers could aid in the collection of classification data. Often, however, people perceive the trophic state concept and the ensuing classification systems as too complex for volunteers. This is simply not true. There are simple approaches to trophic state classification that are not only easy to use, but actually closer to the original intent of the concept. The purpose of this chapter is to give some perspective and guidance to the coordinator with regard to trophic classification systems and to suggest how they can be used in volunteer programs.
understand the complexity and confusion associated with the present
concept of trophic state, it is necessary to begin with a brief
excursion into the history of “trophic state.” We’ll tease apart
some of the numerous strands that are woven together into the present
concept, and suggest that at some of the strands are not as important to
the concept as others, and therefore, can and should be discarded.
at some of the strands are not as important to
the concept as others, and therefore, can and should be discarded.
Einar Naumann, a Swedish limnologist at the University of Lund, Sweden, first developed what we now think of as the trophic state concept in the early part of this century (Naumann 1919). Although trophic state is usually associated with his classification system, it is important to realize that Naumann saw the classification as an artificial outgrowth of a biological reality.
From a 1929 paper, Naumann's
concept of trophic state can be summarized with four clear
propositions about the relationship of the watershed to the
functioning of a lake.
The amount of algae (production) in a lake is determined by
several factors, primarily by the concentration of nitrogen and
2. Regional variations in algal production correlate with the
geological structure of the watershed; lakes in agricultural,
calcareous regions were greener than lakes in forested, granitic
The amount of production in a lake affects the lake biology as
There are certain evolutionary (ontological) connections
between lakes of the various types; lakes become more productive as
trophic state concept, as described by Naumann, began with the
chemistry of the watershed. Nutrients and other chemicals from the watershed, together
with factors such as temperature and light, affected the abundance of
algae (production) in the lake. Production, in turn, affected the
entire biological structure of the lake. A moment’s reflection makes us
realize that this is a very contemporary interpretation of the matrix
of factors affecting a lake’s biology.
statements are neither some fuzzy speculation nor rigid dogmatism
about how lakes should be classified; instead, they are specific,
testable statements about connection between watershed characteristics
and the biology and the ontogeny of lakes. In Naumann’s statements
are the beginnings of ideas of nutrient loading, biomass-phosphorus
relationships, and potential changes in trophic status as a lake ages.
is also remarkable about Naumann’s ideas at this time is his
insistence that there should be a regional approach to the study of
limnology. He wrote in 1929:
advancement of the science of water-types —and of regional limnology
as a whole—is of course dependent upon the collection and comparison
of as abundant data as possible from different countries...In this
respect our special journals could greatly further the advance of
limnology by making it an absolute condition for publication that
contributions should provide the data in question without which, indeed, most such communications are
quite worthless for comparative purposes.”
the sixty years after Naumann, limnologists retreated from comparing
lakes on a regional basis. It might be well for coordinators of
volunteer programs to consider that they are the hope for regional
limnology that Naumann envisioned so many years ago.
Naumann also developed what we now think of as trophic state
terminology, using terms that Weber (1907, as cited in Hutchinson
1969) used to classify the nutrient content of bogs. According to
Weber, oligotrophic bogs were raised bogs where the nutrients
had leached out, while eutrophic bogs were sunken, and
nutrients accumulated in them. Thus the idea that oligotrophic means
“poorly-fed” and eutrophic means “well-fed”
originated from the nutrient condition of bogs, not lakes (Hutchinson
used the terms, but not necessarily Weber’s concept, in classifying
lakes. He based his original trophic classification on the
“quantitative production of phytoplankton” (Naumann 1929).
Oligotrophic lakes were those with low production, “never leading to
a coloring or even a clouding of the water.” In eutrophic lakes,
production attains very high values, “the water being, for the most
part, very strongly clouded or even completely colored.”
related these trophic lake types to the physical and chemical factors
that affect production. These factors included temperature (with which
he divided the world into Arctic and Alpine, Temperate, and Tropical
zones), light, and chemical factors (calcium, humic content, nitrogen
and phosphorus, iron, pH, oxygen, and carbon dioxide). He divided the
possible range of values for each of these factors, which he called
milieu-spectra, into low (oligotypus), medium (mesotypus)
and high (polytpus) “size-classes” or groups. For
example, an oligotrophic lake might have oligotypus values of N and P
and oligo or mesotypus levels of humus (Naumann 1929). Each of
Naumann’s original lake types may have been based on a measure of
production, but he combined the measure of production with a
description of the factor (or factors) that were related to that
production. However, he emphasized that trophic classification was
based on production, not the factor determining that production. He
considered nitrogen and phosphorus to be the primary determinants of
more lakes were studied, it became evident that, although some of
these lakes had production as low as in oligotrophic lakes, the
biological community was distinctly different from the typical
oligotrophic lake. Often these lakes were at the extremes of
axes. Naumann considered these to be new
types when production was largely affected by factors other than
nutrients. The dystrophic lake type, actually described by
Thienemann (1921), had low N and P, but moderate to high content of
humus material. Argillotrophic lakes had low productivity but
the primary trophic factor was the abundance of clay in the water. Acidotrophic
lakes, found at pH values less than 5.5, had as low productivity as
oligotrophic lakes, but a different biological community (Naumann
1931; 1932). A list of many of Naumann’s lake types are given in
Table 7.1. An representation of how we envision Naumann's concept of
the relationship between production and the trophic types is
illustrated in Fig. 7.1.
Germany, August Thienemann simultaneously developed a classification
scheme based on the species of benthic organisms in lakes and the
importance of the hypolimnetic oxygen concentration on their species
composition (Thienemann 1921). It must have seemed reasonable at the
time that these two classifications could be joined, because
Naumann’s eutrophic lakes also lacked oxygen in the hypolimnion and
had distinct benthic fauna (Thienemann 1921). For a while, the
marriage of these systems seemed perfect and the study of trophic
classification grew rapidly (Rodhe 1975).
often happens with classifications of Nature, more and more lakes were
found with characteristics of more than one of the established types.
Hypolimnetic oxygen was supposed to be a primary discriminator between
oligotrophy and eutrophy, but it was found that hypolimnetic oxygen
was not solely dependent on the biological production of the lake, but
was also affected by temperature
and the morphometry of the lake basin. Tropical lakes had low algal
biomass and productivity but still had anoxic hypolimnia. Lakes with
small hypolimnia exhibited anoxia despite low productivity.
with situations such as these intensified the splitting of the
terminology, generating types such as morphometric oligotrophy (Lundbeck
1934, as cited in Hutchinson 1973) for deep lakes with mesotrophic to
eutrophic production but, because of the large hypolimnetic volume,
still had oxygen in the hypolimnion. Järnefelt (1932) proposed the
term “mixotrophy” to designate lakes that had characteristics of
both oligotrophy and eutrophy. Thienemann (1926) went so far as to
state that the lake typology was only applicable in the temperate
regions. The trophic state terminology became increasingly cumbersome
as the number of recognized lake types increased. Classification
became difficult, and most of the terminology finally lapsed into
seeds for the failure of the Naumann-Thienemann trophic
classification were sown almost at its inception. The problems with
the classification were not that a system was not needed nor that the
variables chosen to classify lakes were incorrect. Instead, problems
occurred because: (1) the classifications tried to incorporate all of
the variables, both causal and the biotic and abiotic consequences,
into a single classification; and (2) people assumed that there
existed distinct sets of lakes that could be easily classified.
original idea to classify lakes on the basis of production (biomass)
had both practical and theoretical validity. Production, however
he defined it, was a single
axis that he divided into convenient groupings (high, medium, low
production) and all aquatic bodies could be classified into these
groups. Naumann could map the regional variation in production and
could then ask valid scientific questions as to what factor or factors
might be causing observed differences in production.
began when Naumann linked the production classification with the
causal factors of that production. Again, a useful classification
scheme could be constructed based solely on the factors that affected
or limited production. Indeed, present day trophic classifications
based on phosphorus are remnants of Naumann’s production factor
classification. The classification could have been univariate, based
on the single factor that limited production (nutrients: Oligotrophy -
Eutrophy, clay: Argillotrophy, humic color: Dystrophy, etc.).
Instead, Naumann chose to incorporate all possible factors affecting
production into the classification. He dealt with combinations of
factors by combining the names. For example, an acid lake with high
humic coloring and iron would be called Dys-Acido-Sidero-trophic
(Naumann 1932). If he had found a lake that was high in algae as well,
he might have added eutrophic to that name.
the terminology was not cumbersome enough, production, the supposed
primary standard of trophic classification, was instead the primary
cause of the proliferation problem. If Naumann wanted to classify a
lake on the basis of production and the factor associated with
that production, then a new classification term had to be erected with
each new potential factor affecting production. If he had known of
biological growth limitations by grazing, salinity, water residence
time, or morphometric factors, the classification scheme may have
grown even further.
biological structure, such as the benthic invertebrate community, was
added to the classification scheme, the problems grew even more.
Although the biological community is an important aspect of
limnological ecology, it added a third level of complexity to the
classification. Consequently, the scheme had to consider factors of
limitation, production, and now, biological structure. However,
production is only one of several factors affecting the biological
community composition. Forcing biological structure into the
production index denied the importance of those other factors. The
result was a classification scheme that could not predict structure
from production alone, yet structure was used to predict production.
additional problem came when hypolimnetic oxygen concentration was
added to the classification. As with biological structure,
hypolimnetic oxygen is also affected by factors other than production.
When given equal status, the resulting classification could be
affected by temperature or basin morphometry as much as by biological
net result was (and is) a classification system that is “trophic”
in name only. It involved the degree of production, abiotic elements
that can affect that production, biological structure, and
hypolimnetic oxygen concentration. Thus even in its most
“developed” form, the classification could only be a
listing of general characteristics of generalized lakes. As a result
the classification degenerated into a series of pigeon
holes into which lakes were forced rather than fit.
trophic state terms such as oligotrophy and eutrophy are still
commonly used, and terms such as dystrophy and argillotrophy can
occasionally be found in the literature. Unfortunately, the original
meanings of the terms have become blurred and a variety of definitions
and underlying philosophies are attached to the trophic state terms.
Some of these definitions bear little resemblance to the original
Indices of Trophic State
One lingering artifact of the multiple variable trophic state definition of Naumann-Thienemann is the idea that trophic state, however defined, cannot be measured directly. Instead, one must resort to indices that reflect or correlate with the "true" trophic state of a waterbody. Numerous indices have been proposed, either in attempts to define trophic state or to act as indicators.
contemporary classification schemes use only a single variable to
define the trophic state of lakes. This use of a single variable
simplifies the classification procedure considerably because only one
variable has to be measured. But which variable is the proper one on
which to base the classification? Phosphorus loading, phosphorus
concentration, chlorophyll concentration, algal productivity, algal
biomass (often estimated via Secchi depth and/or chlorophyll
concentration), and hypolimnetic oxygen deficits have all been
proposed at one time or another for a single variable index. Each
could be a valid candidate for trophic state classification and are
briefly discussed below.
(1968a) radically changed our view of lakes when he emphasized the
importance of nutrient inputs from the watershed in the determination
of the concentration of nutrients and, ultimately, the density of
algae in the lake. As we have seen, the idea of the impact of
watershed characteristics originated with Naumann, but, only 50 years
later was Vollenweider able to convince a new generation of
limnologists to look to the watershed to understand the lake.
(1969) and Odum (1969), emphasized the importance of the watershed by
defining trophic state by the loading of nutrients to the lake. For
Hutchinson, trophic state was a description of the potential for a
lake to respond to nutrient loading rather than a description of that
response. A eutrophic system would be a system in which the total
potential concentration of nutrients was high, whether or not it was
expressed in a correspondingly high algal or macrophyte density. Odum
(1969) and Beeton and Edmonson (1972) defined oligotrophy and eutrophy
based on the amount of nutrient loading.
series of loading graphs of Vollenweider (1968, 1975, 1976), appeared
to support the idea that trophic state should be directly related to
loading. Figure 7.2 illustrates how investigators with loading data
are tempted to simply classify the trophic state of a lake.
In this case, trophic state is determined by plotting
"Average inflow concentration," which is calculated by
dividing loading (L) by
water loading (qs), against
hydrologic constraints (water residence time). The
“Permissible” line is the boundary between oligotrophy and
mesotrophy, and “Excessive” line, the boundary between mesotrophy
and eutrophy (Vollenweider 1976).
reality these lines do not represent loading classifications, but
instead predict in-lake phosphorus concentrations of 10 and 20
µg/L (Vollenweider 1968, 1971, 1976). Thus, trophic state categories
are actually based on the predicted phosphorus concentration in the
lake, not loading. The graphs only illustrate the relationship
between loading and predicted lake concentration. In practicality, if
the investigator wanted to know the present trophic state, he/she
could have simply measured phosphorus concentration, and not gone to
the trouble of estimating loading. The value of the graphs and the
associated models is in the prediction of trophic state if
nutrient loading is changed, not the estimation of present trophic
from the misuse of the Vollenweider graphs by some to estimate the
trophic state of a lake, nutrient loading has not been used
extensively to define trophic state. Although the impact of nutrient
loading on lake conditions cannot be understated, the use of nutrient
loading to define the “state” of the lake seems inappropriate.
Loading can only, as Hutchinson implies, define the potential state of
the lake. While potential nutrient concentration is an important
measure, it does not provide the information about the lake that has
attracted limnologists to trophic state classification.
concentrations of nutrients, especially phosphorus, have probably been
the most popular variable for single variable trophic state indices.
The dominance of trophic state designations based on phosphorus
concentration rather than nitrogen is probably the result of the
widespread belief that phosphorus limits algal growth in most lakes.
The impact of Vollenweider's loading models on the field of predictive
limnology and management also helped emphasize the role of phosphorus.
phosphorus limited lakes, there should be a strong relationship
between phosphorus (usually measured as total phosphorus) and plant
(algal) biomass. Because of this relationship, phosphorus can be used
as an estimator or predictor of production. If
phosphorus concentration is used to predict trophic state (biological
production), nutrient concentrations are being used in the manner that
Naumann originally intended.
idea that phosphorus can predict trophic state (production) must be
clearly differentiated from the use of phosphorus as the definition of
trophic state. While Naumann considered phosphorus and nitrogen to be
the primary factors that determined trophic state, he initially based
trophic classification on the biological condition of the lake, not
the cause of that condition. The trophic state of a lake is a
biological condition caused by various factors such as nitrogen
and phosphorus, but also potentially affected by pH, turbidity, color,
phosphorus is used instead of production to define the trophic state
of a lake, the program coordinator must be willing to forgo any
accurate inference of the biological condition in a large number of
lakes and reservoirs. Even the best of the
chlorophyll-phosphorus models has considerable variation. More
importantly, in some lakes, and certainly in many reservoirs, algal
chlorophyll is not related to phosphorus concentration. In turbid
lakes and reservoirs, for example, chlorophyll cannot be predicted
with any accuracy because much of the phosphorus is probably attached
to the non-algal particles and is unavailable for algal growth
(Carlson 1992). If the biological condition of the lake is of any
importance to the classification or management of a lake or reservoir,
there is no justification, neither historical nor practical, to use
phosphorus as the defining variable of trophic state.
productivity, the rate at which light energy is incorporated into
plant cells, has long been a standard of trophic state classification.
Åberg and Rhode (1942) and Rhode (1969) interpreted Naumann’s
original trophic state definitions to be the biological productivity
of a lake rater than production (the amount of plant material in the
lake). Certainly, the biological dynamics of a lake and, to some
extent, its biological structure, are dependent on productivity.
productivity is an important measure in lake evaluation, and has some
historical claim as an early trophic state definition, it does have
methodological drawbacks that argue against its use as the major
estimator of trophic state. Traditionally, productivity requires an
estimate of annual productivity, the obtaining of which requires
extensive year-long sampling.
sensitivity of available methods for data collection is also a
problem. The oxygen evolution technique is relatively easy to use and
requires little equipment, but is not as sensitive as the
Carbon‑14 technique, which requires radioactive materials and
skilled personnel. There is also the concern about radioactivity in
the environment, which makes the ability to use C‑14 in the
field difficult if not impossible.
Productivity is also subject to interpretational problems,
because, when it is expressed on an areal basis (mg/m2/yr),
productivity can be as much influenced by non-algal attenuation
of light as by the rate of plant photosynthesis. When productivity is
expressed on a volumetric basis (mg/m3/yr), the effective
volume (epilimnetic, photic, or whole lake volume) becomes problematic
biomass is the weight of the algae in the lake and should be expressed
as concentration. It can be reported as grams of dry weight, grams of
carbon, or as biovolume (the total volume of plant material per volume
of water). Unfortunately there is no test that can, without error,
measure what we glibly call biomass. Variables such as total
particulate carbon or total suspended solids can neither differentiate
living from non-living materials such as sediments, clays, or
detritus; nor bacteria from the algae. Consequently, each of these
variables becomes an estimator of biomass, but none can be used to
define it. Biovolume does not suffer from problems of interference,
but it does involve a time-consuming measurement process which
requires some expertise. It must also be converted into carbon or mass
units, using some conversion factor.
other variables commonly used as algal biomass surrogates are
chlorophyll concentration and Secchi depth. The concentration of the
plant pigment, chlorophyll, is often used as an indicator of trophic
state. Chlorophyll is popular because it is relatively easy to measure
and does not suffer from the interferences (detritus and
non-algal particulates) found in the other variables. Since
chlorophyll is also integral to photosynthesis, chlorophyll serves as
the link between productivity (rate of carbon incorporation) and
has drawbacks as a biomass surrogate, however. Perhaps the greatest
drawback is that the amount of chlorophyll in an algal cell may vary
considerably, depending on the physiological condition of the cell or
the plant species. Cells that are subject to low light conditions will
have more chlorophyll in them than cells exposed to high light.
Different species of algae will contain differing amounts of
chlorophyll (Tolstoy 1979). Despite variation, relationships between
chlorophyll and algal biovolume and algal density exist, suggesting
that chlorophyll changes as cell density changes (Carlson 1980;
Watson, et al. 1992).
depth has also been widely used as a surrogate estimator of trophic
state. Transparency itself is not considered to be a definer of
trophic state, but transparency is influenced by algal density, and
therefore can be used as an inexpensive surrogate for algal biomass.
Secchi depth correlates best with algal biomass (as measured by
chlorophyll) in non-colored, non-turbid situations where
algae dominate the attenuation on light. However, it is subject to a
number of interferences and methodological problems (See Secchi Depth
chapter). Thus Secchi depth should not be used exclusively when better
methods are also available.
major oversight in the use of algal biomass as the definer of trophic
state is that it ignores the presence and importance of aquatic
macrophytes in the determination of the trophic state. Their omission
may seriously underestimate the total plant biomass in a lake. A
method to remedy this problem will be discussed later.
presence or absence of oxygen in the hypolimnion of lakes is often
used as a major aspect of trophic state classification. When
Naumann’s and Thienemann’s classification schemes were combined,
hypolimnetic oxygen became one of the defining characteristics of
trophic state. As mentioned earlier, problems arose when morphometric
factors were found to also play a major role in defining the presence
or absence of hypolimnetic oxygen during the summer. Hypolimnetic
temperature was also found to regulate the rate of oxygen depletion.
Dissolved organic compounds also contributed to the depletion of
oxygen, even in lakes of low productivity, and thus became one of the
defining characteristics of dystrophy (Thienemann 1921; Naumann 1932).
limnologists would use the presence or absence of oxygen as the sole
delimiter of the difference between oligotrophic and eutrophic lakes,
regardless of the productivity or production within the lake. While it
is true that oxygen depletion can have a dramatic effect on the biota
and chemistry of the lake, it would be better to consider hypolimnetic
anoxia as a possible result of the lake's trophic state, not the
definition of it. Certainly, anoxia can result from increased
productivity, but the relationship between the amount of productivity
and anoxia is modified by the volume of the hypolimnion and the
temperature of the hypolimnetic water. Volume and temperature have
nothing to do with the trophic state concept, yet are major
determinants of hypolimnetic anoxia. Thus the term, trophic state,
loses much of its meaning if temperature and volume become its
determinants. It seems best, therefore, to regard anoxia as a
consequence of eutrophication, relying on Naumann’s, rather than
Thienemann’s trophic concept.
the combined Naumann-Thienemann trophic state definition, the
lines between morphological factors, chemical factors, and biological
structure became increasingly blurred. Naumann’s original emphasis
on biological production became lost in a list of “trophic state”
variables, some or all of which were used to classify lakes. Some of
the modern treatments of trophic state continue this
“combination-of-ingredients” approach to trophic
classification, declaring that trophic state is a complex aggregate of
physical, chemical, and biological variables that can only be dealt
with in a multi-variate manner and classified in probabilistic
terms. Some of the most prevalent forms of multi‑variable
classifications are discussed below.
simplest forms of these multifaceted definitions are those that
combine a nutrient causal factor, usually phosphorus, with a
biological effect factor, such as algal biomass. For example, a
eutrophic lake would be a lake with high algal chlorophyll caused
by high phosphorus concentrations or high nutrient loading. In
these definitions the process of eutrophication, rather than trophic
state, is being defined. For example, eutrophication might be defined
as increased nutrient loading (cause) which results in increased
biological productivity, plant biomass, or hypolimnetic anoxia
(effect). Carlson (1984) points out that these cause-effect
definitions suffer because, in some cases, the effect is not
necessarily the result of the defined cause. Zooplankton grazing or
short water residence times can lower algal density despite high
nutrient loads. If a lake becomes shallower with time, and macrophytes
become abundant, has the lake undergone eutrophication if plant
productivity has increased but external loading or internal nutrient
concentrations have not?
linking of cause and effect triggered the proliferation of Naumann’s
terminology, and the weaknesses and pitfalls of that approach remain.
Faced with situations where the effect occurred independent of changes
in the causal factor, the investigator must either ignore the
contradiction, choose to classify on either the cause or the
consequence, or, as did Naumann, add new names to the system. None of
these options seems desirable.
Based on Lake Types
unfortunate remnant of the Naumann-Thienemann trophic state system is
the belief that lakes can be divided into distinct classification
groups or “lake types.” Lake typology reflects the philosophy that
lake types are distinct, potentially separate entities, each able to
be classified according to its own characteristics (Carlson 1979).
These characteristics may exhibit variation within each lake type, and
may even overlap between types, but the types are considered largely
distinct from one another.
typological classifications use a table of characteristics that would
be expected for each lake type. An example of a qualitative list of
characteristics is given in Table 7.2. This list provides a general
impression of the trophic state of a lake or reservoir, but is
difficult to use. Quantitative tables also exist, using either
distinct boundaries between trophic states or overlapping ranges of
values, as in Table 7.3.
the variables on a quantitative or qualitative list are not given any
priority; a lake may be classified as oligotrophic by one variable and
mesotrophic or eutrophic by another. This situation often happens in
deep lakes where the hypolimnion contains oxygen but there are visible
algal growths in the upper waters, or in smaller lakes, where the
water may be clear, but the hypolimnion is anoxic.
indices are assumed to work no matter what the kind of lake or where
it might be located. Even Naumann and Thienemann recognized that lakes
vary geographically in their characteristics. As mentioned earlier,
Thienemann initially thought that the classification system was only
suitable for lakes in temperate climates.
classification systems produce only a qualitative label for the
reservoir, however. Labels do not lend themselves to quantification,
and, therefore, to prediction. Thus, the trophic designation becomes a
dead end, rather than a gateway into predicting the other aspects of
the reservoir’s chemical and biological condition, as was envisioned
labels also suffer because the labels do not recognize variations
within the type. There is a great deal of variation and change
possible in any variable, be it phosphorus, chlorophyll, or biological
structure, that is incorporated within the single trophic state
designation of “eutrophic.” Under the present trophic state
classification system two lakes of the same trophic label might have
chlorophyll concentrations that vary by two or four‑fold. Most
limnologists don’t recognize this as a problem, so how could
volunteers be expected to know that these lakes might function very
solution is to add more trophic categories, such as
ultra-oligotrophic and hypereutrophic, but this adds more names
to memorize and to characterize. Adding more categories also
undermines the concept that lakes are distinct types, and tacitly
acknowledges that the names are being used to arbitrarily divide a
approaches also ignore all of the other trophic types that Naumann
erected. Lists such as that given in Table 7.2 could not classify a
lake with a 10 cm transparency filled with silt nor a lake with highly
colored water. Of course the list could be expanded to include other
lake types, but the list approach was tried by Naumann and history
discarded it. It does not seem necessary to recycle an approach proven
to be inadequate .
quantitative typological classification system was developed by a
study group sponsored by the Organization for Economic Cooperation and
Development (OECD). The study group apparently believed that lake
types did exist, but that there was variation in the values of
variables associated with any lake type. They produced a
classification system based on the probability that a lake or
reservoir will have a given trophic state (Fig. 7.3). For example, a
lake having a total phosphorus concentration of 10 mg/m3 in
the epilimnion would have a 63% percent probability of being
oligotrophic, a 26 percent chance of being mesotrophic, and a 1
percent chance of being eutrophic (Vollenweider and Kerekes 1980).
Contrast this approach with that of Vollenweider (1976), where 10 mg/m3
is used as an absolute boundary between oligotrophy and mesotrophy.
assumed advantage of the probabilistic approach is that responses to a
given variable, such as phosphorus, will vary
lake-to-lake, and therefore prediction of its trophic
state is best stated in probabilistic terms. This approach is
analogous to the difference of predicting a single phosphorus
concentration for a lake from a loading model and the presenting the
probability of a predicted nutrient concentration.
there are several problems with the OECD classification method. First,
the system is based on the premise that distinct lake types exist. The
probabilistic advantage exists only if distinct trophic types actually
exist. However, the criterion or criteria that designate a given
trophic type are not stated. What does an oligotrophic lake look like
at high phosphorus concentrations? Is it called oligotrophic because
of low biomass, productivity, or, because oxygen remains in the
original designations were based on the opinions of experts, not on a
single or even multiple quantitative criteria. The ranges of the
curves were obtained by deriving a normal curve from the mean and
standard deviations of opinions of a group of experts; no lake may
ever exist at the extremes of each trophic state. Setting
probabilistic limits may seem more realistic than absolute limits, but
only if trophic states are real, not arbitrary divisions of a
second problem with this approach is that, as with a qualitative
classification system, no effort has been made to first correlate the
different variables to provide a single trophic designation. It is
possible that a lake may be classified as eutrophic if phosphorus is
used as the defining variable, and mesotrophic or even oligotrophic if
chlorophyll is used. This dual classification is not due to
variability in the values of each variable, but to the fact that the
authors of the index did not examine possible empirical relationships
the end product of the index is a trophic state label. Apparently all
the sources of variation are incorporated in the probability
distribution; no effort is made to identify and separate out the
sources. Therefore, this type of
index has little predictive ability, especially outside the
geographical region where it was designed. The index suffers from all
the problems listed above for qualitative indices.
indices attempt to maintain the multiple variable aspect of the
Naumann-Thienemann concept while adding a quantitative trophic
state designation to the index. This is done by combining more than
one variable into a single index value. The multi-variate
indices of Brezonik and Shannon (1971) and the EPA trophic state index
(EPA 1974) utilize a number of variables related to trophic state, yet
deviate from the classical interpretation in that they produce a
continuous, numerical classification rather than typological
definitions of trophic state now exist in the literature. There are so
many, in fact, that the entire concept is at risk of becoming a
“non-concept.” Carlson (1984) comments that this situation
would be a real loss to limnology, because the concept, if not the
classification, still has the potential to communicate valuable
information to scientists and to the lay public. Perhaps even more
importantly, it has the ability to aid in the organization of
thoughts, ideas, and research about lakes. There are good reasons to
think that trophic state remains a critical organizing concept in the
study of lakes.
asking questions about fish production without referring to the base
of food chain, or discussing restoration without reference to the
watershed. Most investigators will preface their studies with a
description of the trophic state of a lake, assuming that all the
readers will understand the meaning of the term. Trophic
classification is a necessary statement that allows us to locate the
lake in the production continuum. From that location we can make
predictions of structure and function of that system. Without that
location and the ensuing ability to estimate further attributes of the
system, each lake becomes an independent entity, devoid of all
connection to previous information and knowledge of other lakes, or
even previous studies. This certainly is not the way most limnologists
think of lakes and limnology.
attempt to resurrect and modernize the Naumann-Thienemann trophic
concept will ultimately be met with ever-increasing frustration
similar to that found by past investigators. Some classifiers have
chosen to ignore the possible variations in biological structure
caused by factors other than nutrients, producing a single-axis
trophic scale based on nutrients alone. Unfortunately, the biological
structure of all lakes does not respond in a linear manner to nutrient
additions. If the investigator chooses to include other abiotic axes
as well, he risks the proliferation of lake types, overlapping and
contradictory classifications, and a system as cumbersome and as
susceptible to collapse as was the Naumann-Thienemann model.
the risk of producing just such a trophic classification system, below
are several statements which may be controversial, but might lead to a
usable and useful classification system.
There are no such thing as lake types.
typology is a historical artifact that has no reality in lakes. No
study supports the concept that lakes or their biota have a distinct
identity or wholeness that separates one lake type from another. Most
abiotic variables change continuously along gradients, and production
and the biological structure change continuously as well. The
propensity to lump lakes into groups is a quirk of the human mind, not
a necessity determined by Nature.
presence or absence of hypolimnetic oxygen comes closest to a single
separating variable because along the entire axis of possible nutrient
and algal concentrations, anoxia develops relatively rapidly (within
one doubling of phosphorus concentration). However, even the
development of hypolimnetic anoxia is a continuous, rather than a
discrete process, appearing to be abrupt only when compared to the
entire spectrum of possible nutrient or biomass concentrations. In any
given lake, the process is defined as the rate at which oxygen is
depleted, not the mere presence or absence of oxygen.
a continuum into subgroups is done for convenience. However, once
classes have been established, they take on a life and reality of
their own, as happened with Naumann’s classification. It would be best to abandon the trophic terminology all together, relying on
quantitative classifications instead. It is more difficult to ascribe
groupings to a numerical scale.
typology and trophic classification are not the same thing.
the original trophic classification developed, the original
classification of production became less and less obvious as the
principal variable of interest. For the most part, the classification
exercise became one of classifying lakes rather than classifying the
amount of biological production. All the factors that affect
production became incorporated into the classification; nutrients,
light, morphology, turbidity, location, etc.
can be described in many ways, including depth, thermal structure,
water chemistry, size, etc. Trophic state, however defined, is only
one possible manner of classifying lakes. In retrospect, Naumann’s terminology
should have been specific to production or to the factors that limit
production, not incorporate both. An acid lake is only acidotrophic if acid limits
production. A turbid lake is argillotrophic only if the turbidity
limits plant biomass. Unfortunately, Naumann’s classification seemed
to confuse, then fuse, trophic description with the description of the
lake. If trophic classification is looked upon as reflecting one
particular (and important) aspect of a lake rather than as some sort
of total classification, the trophic concept can be kept simple and in
is the simplest and most useful definition of trophic state.
of what is written in this chapter alludes to the fact that no single
variable or even concept has exclusive historical claim to be the
“true” definition of trophic state. Even in Naumann’s time, and
apparently with his blessing, the concept changed and became more
elaborate. The fact that the classification finally collapsed
indicates that the scheme had become too elaborate and inclusive. It
no longer reflected a chief purpose of classification—utility. Many
biological classifications, including binomial nomenclature, are, at
least in part, artificial. Being largely a construct of the human
mind, they are simple tools in organizing and communicating ideas.
Trophic state definitions are no different; they must organize
knowledge and they must communicate that knowledge.
has already been explained why nutrients should not be used as the
basis for a trophic state definition. It would not be historically
accurate, but more importantly, neither phosphorus nor nitrogen is the
primary object of interest in lake classification or management. They
only gain their importance as they relate to or affect the biological
situation in the lake. The fact that a lake has high concentrations of
phosphorus is only of interest if that phosphorus can determine the amounts of algae or macrophytes.
based on either biomass or productivity can draw upon a long tradition
of classifying aquatic as well as terrestrial ecosystems by amount of
organic matter in the system and/or the rate of entry of energy into
the system. The reasons why productivity would not be the best trophic
state variable have already been explained; it is difficult to measure
and to interpret.
trophic state definitions are consistent with systems terminology’s
use of the term “state,” as a measure of the amount in a system at
a given point in time. Although the term “biomass” itself must be
operationally defined, it can be estimated by a number of techniques,
from Secchi depth to adenosine triphosphate (ATP) analysis. With such
a variety of available techniques, a technique appropriate to each
budget or situation can usually be found. Biomass also lends itself to
multi-lake surveys, where there is often insufficient time to do
intensive productivity analyses. Finally, biomass is an approximate
measure of the problems that plague lakes. Few citizens complain about
the productivity of their lake and fewer yet lodge complaints about
phosphorus. A biomass-related trophic state definition places
the emphasis of the classification on the problem (e.g., too much
algae or too many macrophytes which, in turn, interfere with lake
uses) rather than on any potential cause.
Eutrophication is nothing more (or less) than the movement of a
lake’s production along a continuum in a direction from oligotrophy
Naumann’s trophic state classification, the terms oligotrophic and
eutrophic marked two classes of lakes along the nitrogen and
phosphorus axis. As a lake became eutrophic, the process was called
eutrophication. Conversely, if a eutrophic lake became oligotrophic,
the process could be called oligotrophication. Eutrophication
is not a mysterious process or even one necessarily linked to nutrient
change. It is simply a directional movement.
production might be increased or decreased by factors other than
nutrients, it is more appropriate to use production as the central
axis rather than nutrients. Because production is most often related
to changes in nutrient status in the lake, eutrophication and
oligotrophication are processes often caused by changes in the
lake’s nutrient content, and, ultimately, nutrient loading to the
lake. However, the change could just as well take place if another
limiting factor, such as turbidity, were removed, as long as
sufficient nutrients were available to allow increased plant
production. Eutrophication would have occurred even if there had been
no change in loading, or even nutrient status.
structure of lakes is affected by many factors other than
nutrients and should not be used to define trophic state.
production, the biological structure of a lake is much more
susceptible to change along gradients other than nutrients. As
mentioned above, one mechanism to maximizing production and
productivity is a change in the dominant species to those best suited
to the ambient conditions. Changes in species can be expected as a
lake becomes acidic or colored with humic substances. In other cases,
intolerance to prevailing conditions will eliminate whole groups of
organisms, as was found in acidic lakes, where vertebrates were
largely missing. In these instances, whole trophic pathways will be
altered or eliminated. The same alteration of food webs may also be
found in other environments such as turbid reservoirs and lakes.
trophic state were defined on the basis of biological structure rather
than on production, it might be expected to change as a function of
the intensity of a number of environmental factors. Change pH,
nutrient loading, hypolimnetic anoxia, or salinity, and the biological
structure (and therefore trophic state) will be expected to change as
structural changes will not fit neatly into lake types because changes
will probably occur gradually along any one or more environmental
axis. Any sort of typological classification would be impossible; the
name game would be overwhelming. Probably some sort of multivariate
quantitative classification would be most useful. Whatever the
approach that is used, the ability to classify the biological
structure of lakes is clearly needed.
structure has a legitimate and compelling claim to the concept of
trophic state. Although not the original trophic state definition,
biological structure gained importance in later classifications,
especially after the merger of the Naumann and Thienemann
classifications. Naumann considered that many of the lake types, such
as acidotrophy or argillotrophy, had equal productivity but distinctly
different biological communities.
a subtle way, Naumann made a distinction between production and
structure. He split the classification into two typologies, one
describing the water (Wasser) and the other the water body (Gewasser).
In the first he placed production, in the second, biological
structure. What he did was to distinguish between variables such as
production and productivity and those relating to biological species
(structure). This distinction is useful because they are really two
distinct but related aspects of biological systems.
and productivity relate to the amount and rate at which energy enters
the lake system. They share with other variables, such as nutrient
concentration, the attribute that they are conservative, that one can
always account for the income, outgo, and location of every
kilocalorie or gram. People can write energy or nutrient budgets for
them and predict how much energy or nutrient will be used in the
future. They are, however, fundamentally different from biological
structure. Biological structure is based on biological species. A
species becomes present in the community when a single organism
arrives, but the loss of a single individual does not mean the species
is absent. For a species to be absent again, it must go extinct. A
species cannot be weighed; it may consist of a single individual
weighing a kilogram, or the total weight of millions of individuals of
a species may weigh only a thousandth of a gram.
are a member of a category called information. Information is non-conservative;
that is, you can’t weigh or account for information in the manner
you can with materials and energy. Two cents worth of information can
be given to one person or a million people, each receives the same
amount of information. A single individual of a species brings far
more than two cents worth of information into an ecosystem. That
information is present whether there be one or a million individuals
of that species. This non-conservative nature of information
makes it difficult, if not impossible, to predict species structure from energy or
material variables alone. A kilogram of organic material may contain a
single or a thousand different species, arranged in an almost infinite
number of trophic web arrangements.
important yet, biological structure has the unique ability to adapt,
evolve, and change to fit the surrounding environment. Given time, it
is theoretically possible for life to evolve for the maximum
utilization of nutrients and energy even in the most extreme
broad thermodynamic constraints, however, evolution can produce a
dazzling kaleidoscope of possible biological communities.
biological structure can be so diverse, it could be successfully
argued that each lake ecosystem is to some extent comprised of a
unique community structure. If each lake is different from the next,
then any classification ultimately will be frustrated by this range of
possibilities. Alternatively, classification may attempt to force the
diversity of structure into an artificial classification scheme,
overlooking the small differences, preferring to see the similarities.
structure classification is distinctly different from what Naumann
originally intended (i.e., the linking of watershed characteristics to
the biological production of the water). The original trophic scheme
was deeply involved in the factors of production, not structure. It is
to Naumann’s credit that he insisted that production would, in turn,
affect structure, and ultimately the ontogeny of the lake itself.
Classification based on biological structure would have to include
numerous measures that affect biological structure, but which are
unimportant to Naumann’s concept of factors that affect production.
the final analysis, it is recommended that the amount of plant production (biomass) and not
biological structure be used as the definition of trophic
state. It has historical consistency, is relatively easy to measure,
and is a measurement that conveys meaning to limnologists, lake
managers, and the public. Biological structure should be considered as
a separate classification. Structure is important and far more
vulnerable to environmental stresses than production. The ability to
classify and track structure is an important aspect of limnology
because structural changes may ultimately affect function. It deserves
more attention, but it should not be called trophic state. Call it
biodiversity, biological structure, biotype, or whatever, but just not
coordinator of a volunteer program is in a unique position to
understand the need for a clearly‑defined, useful, and
communicable definition for trophic state. The coordinator serves at
least three basic constituencies. First, monitoring programs are often
mandated by the state governments. In some states coordinators are
required to obtain certain information to satisfy 305b reporting
requirements and to qualify for 314 Clean Lakes funds. The trophic
state determination is needed for both programs. The more complex the
definition of trophic state, the more information that will be
required to classify lakes. If the definition is too complex, data
requirements will be beyond the scope and budget of most volunteer
volunteers expect information, not labels. They will be poorly served
for their time and effort if all they receive is a comment that their
lake is eutrophic. What does this term mean to the volunteer? Is the
lake in good or bad condition? Should it be restored or protected?
What is the prognosis for recreation and other uses? It would be
better if the definition reflected a single aspect of the lake very
well rather than present a combined variety of factors that does not
convey quality information.
the coordinator, or those that use the volunteer data, must make
management decisions. Decisions require information. However, if the
goal of the program is serving the data collection needs of a
classification system, that classification had better provide more
information than just a name. Certainly trophic state is only one facet
of what should be called water quality, but it would be ideal if the
definition described an important aspect of a management program.
trophic state model presented above is amenable to a tiered
level-of-effort approach which is compatible with the
funding and goals of many volunteer programs. Most programs begin with a
Secchi depth monitoring program because it is easy to train volunteers
in the disk’s operation and because Secchi disks are relatively
inexpensive. Since Secchi depth is also an indicator of algal biomass,
it is also possible to make estimates of trophic state, albeit with
a program matures, chlorophyll pigments and total phosphorus are added
to provide a better estimate of biomass. Later in this chapter, a method
will be given that uses Secchi depth, chlorophyll, and total phosphorus
to make inferences about potential limitation. Addition of total
nitrogen and estimates of macrophyte biomass would complete this suite
of variables that would completely estimate trophic state, as defined by
plant biomass, and the nutrient factors that often cause that trophic
sampling by volunteers could provide information on biological
structure. Samples of macrophytes, phytoplankton, and zooplankton could
be obtained by the volunteer and shipped to a laboratory for
identification. Creel censuses or careful records of fishing success by
the volunteers might give some indication of fish community structure.
these data, a picture of a lake’s trophic and biological structure
will emerge. From that picture, it may be possible to ask questions of
causation, and a watershed monitoring program could be initiated.
Volunteers could do much of the work in a watershed monitoring program
and the information gathered could be very useful in managing the lake.
state determination is an important aspect of lake surveys. Trophic
state is not the same thing as water quality, but trophic state
certainly is one aspect of water quality. Of particular interest for
volunteer programs is that trophic state determination can be
performed by volunteers and it can be immediately useful to them.
Several recommendations can be made with regard to the use of trophic
state classifications in volunteer programs.
Use the simplest definition of trophic state: neither you nor
the volunteer will benefit by making the concept complex or somehow
2. The recommended definition is plant biomass: it is historically correct, simple to measure, and simple for a volunteer (or manager) to understand. It also can be predicted from nutrient models and can be used to predict other biological characteristics.
The trophic state index of Carlson (1977),
which is based on plant biomass, is recommended as the
simplest method of relating trophic state concepts to volunteers.
4. Remove the mystery from the term eutrophication. Rather than linking the process to nutrients, which can cause all sorts of interpretational problems, simply define it as a movement of the lake’s trophic state in the direction of more plant biomass. The definition is simple and far more functional than any other definition.
Use trophic state as a teaching tool. Discuss with the volunteers
all the possible factors, not just nutrients, that could make a lake
more eutrophic. For example, explain that the deposition of erosional
materials will cause the lake to become shallower, and therefore
enhance macrophyte growth, thus affecting the total amount of biomass.
Discuss the ramifications of change in plant biomass, how it affects
hypolimnetic oxygen and fish species and its possible effect on food
chains and recreational potential.
Be very careful about using quality terms when speaking of
trophic state. Even your own perception of quality is affected by your
background and education. Be sensitive to the fact that not all users
will want the same type of water quality that you do. Not everyone
considers the ideal lake to be clear. Always be sensitive to the
background and needs of the users.
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